El Niño and the Indian Monsoon

El Niño is the warm phase of the El Niño-Southern Oscillation, a coupled ocean-atmosphere phenomenon in the tropical Pacific. It involves sustained, large-scale warming of the central and eastern equatorial Pacific, accompanied by changes in winds, pressure and tropical rainfall that influence the Indian southwest monsoon through atmospheric teleconnections.

Physical mechanism linking El Niño and the monsoon

El Niño influences India by reorganising tropical atmospheric circulation through a teleconnection.

• Normally, easterly trade winds push warm surface water towards the western Pacific, where rising warm, moist air supports strong convection.
• During El Niño, the trade winds weaken, allowing warm water and convection to shift towards the central and eastern Pacific.
• This eastward shift alters the Walker circulation and redistributes zones of rising and descending air across the tropics.
• The resulting circulation anomalies can favour descending air over parts of the Indian region, suppressing cloud formation and rainfall.
• Associated changes in pressure gradients and winds can weaken monsoon circulation and moisture transport towards India.
• The Southern Oscillation is the associated atmospheric pressure fluctuation; together, the oceanic and atmospheric components constitute ENSO.

Nature of the rainfall influence

The El Niño-monsoon relationship is probabilistic, not automatic: every El Niño does not cause a drought.

• Rainfall may become uneven, with deficient conditions in some regions but weaker or offset anomalies elsewhere.
• Weak or delayed early-season rainfall and longer dry spells may interrupt the progression and continuity of monsoon rainfall.
• Seasonal totals can conceal intra-seasonal variations because the timing and sequencing of wet and dry spells also determine monsoon impacts.
• Indian Ocean conditions, including the Indian Ocean Dipole, and intra-seasonal monsoon variability can reinforce or offset El Niño's influence.

Agricultural and economic consequences

Agricultural effects depend on rainfall distribution, irrigation coverage, crop choice and adaptation rather than only the seasonal rainfall total.

• Weak or delayed rainfall can postpone kharif sowing and reduce planted area, especially in rainfed farming.
• Deficient rainfall lowers soil moisture and can impair germination, crop growth and yields; dry spells at critical stages can reduce output.
• Reduced reservoir inflows and groundwater recharge can affect irrigation during the remaining kharif season and subsequent rabi crops.
• Widespread crop losses can reduce rural incomes and raise food prices.